1. Technical Field
This disclosure relates generally to an image forming apparatus, and more particularly, to an image forming apparatus using a recording head including a liquid ejection head that ejects liquid droplets.
2. Description of the Background
One example of related-art image forming apparatuses such as printers, copiers, plotters, facsimile machines, and multifunction devices having two or more of printing, copying, plotting, and facsimile functions is an inkjet recording device employing a liquid ejection recording method. The inkjet recording device includes a recording head that ejects droplets of a recording liquid such as ink onto a sheet of a recording medium while the sheet is conveyed to form an image on the sheet.
Examples of the inkjet recording device include a serial-type image forming apparatus, in which the recording head ejects liquid droplets while moving in a main scanning direction to form an image on the sheet as the sheet is moved in a sub-scanning direction perpendicular to the main scanning direction, and a line-type image forming apparatus equipped with a line-type recording head that ejects liquid droplets and does so without moving to form an image on the sheet as the sheet is moved in the sub-scanning direction.
A maintenance mechanism that maintains performance of the recording head is essential for the image forming apparatus employing the liquid ejection recording method. One of the functions of the maintenance mechanism is to discharge bubbles, foreign substances, coagulated ink, and so forth present in the recording head through nozzles in the recording head in order to prevent irregular ejection of the ink from the nozzles in the recording head.
In addition, a full-color image forming apparatus that forms full-color images using the liquid ejection recording method generally includes two separate recording heads, that is, a recording head that ejects black ink droplets (hereinafter referred to as the first recording head) and a recording head that ejects color ink droplets (hereinafter referred to as the second recording head). In such a full-color image forming apparatus, not only black ink but also color ink is ejected for maintenance of the recording heads even when monochrome printing is performed using only the first recording head, causing a waste of color ink and a concomitant cost increase.
In order to solve those problems, some image forming apparatuses deploy separate carriages for the black and color inks. That is, they include a first carriage mounting a first recording head that ejects black ink droplets and a second carriage mounting a second recording head that ejects color ink droplets. The first and second carriages are separatably dockable with each other.
For example, the first and second carriages may be selectively dockable with each other via a scanner (or a carrier) using a gripper. In order to prevent looseness between the first and second carriages docked with each other via the scanner, shielding plates are respectively provided to the first and second carriages and the scanner. Accordingly, a correction amount for controlling relative positions of the first and second carriages is obtained based on the timing with which each of the shielding plates shields light emitted from a home position sensor provided at a certain position in the image forming apparatus.
In another approach, a lock is provided to the scanner to engage with a gripped portion provided to each of the first and second carriages to lock the scanner and the first and second carriages together.
However, in the above-described configurations, the first and second carriages are docked with and separated from each other through an intermediate member such as the scanner and the gripper. Consequently, the accuracy with which the relative positions of the first and second carriages are secured is decreased due to the use of the intermediate member, thus degrading image quality of full-color images.
Further, repeated docking and separation of the first and second carriages change the relative positions of the first and second carriages. Consequently, target positions to which the ink droplets are ejected from the first and second recording heads onto a recording medium (hereinafter referred to as landing positions) are shifted between the black ink droplets ejected from the first recording head and the color ink droplets ejected from the second recording head, thus degrading image quality of full-color images.
Thus, the relative positions of the first and second carriages are not accurately corrected only by obtaining the correction amount described above.
Meanwhile, there are also image forming apparatuses that correct a timing of ejection of ink droplets from recording heads (hereinafter referred to as an ejection timing) in order to prevent the shift in the landing positions between the black and color ink droplets. Specifically, the image forming apparatus forms an adjustment pattern and reads the adjustment pattern using an optical sensor to correct the ejection timing of the black and color ink droplets, thereby reducing color shift during full-color image formation.
However, although the landing positions can be corrected when the first and second carriages are docked with each other, the above-described image forming apparatus cannot handle variation in the relative positions of the first and second carriages caused by repeated docking and separation of the first and second carriages. As a result, because the relative positions of the first and second carriages may be changed by repeated docking and separation of the first and second carriages, the adjustment pattern must be formed each time the first and second carriages are docked with each other in order to calculate the correction amount.